The current problem with cryptographic communications is due to the fact that the message transmitted from point A to point B can be copied in the interval that separates the two points. To overcome this problem, sources are used that emit only a single photon at a time to transmit binary random key type messages. An example of this type of use is described in the article by J. Rarity et al. entitled “Single Photon Sources And Applications”, Confined Photon System Fundamental And Applications, Springer Verlag LNP 351, pages 352-365.
However, the remaining problem is to make such a source that emits only one photon at a time. At the present time, there are two major techniques. The first is based on extreme attenuation of a laser source. The second is based on the use of a microcavity type spectral filter on “quantum box” type emitters.
The first approach has the disadvantage that the probability of two photons being emitted instead of one is not negligible. Consequently, this makes it possible to copy these photons since the two photons will have the same polarization.
The second approach, for example, described in the article by E. Moreau et al. entitled “Single-Mode Solid-State Single Photon Source Based On Isolated Quantum Dots In Pillar Microcavities”, Applied Physics Letters, Vol. 79, No. 18, pages 2865-2867, requires the use of direct gap materials type II-VI or III-V (a gap is the difference between the conduction band and the valence band), and these materials are extremely difficult to work with.